A pearl is a lustrous concretion produced by certain bivalve molluscs (muscles and oysters) and valued as a gem. Pearls consist almost entirely of nacre, which is the substance forming the inner layers of the mollusc shells. Nacre, known as mother-of-pearl, is composed primarily of aragonite crystals. The pearl is an abnormalgrowth resulting from the invasion of the body of the mollusc by a minute parasitic worm (it was previously believed to be invasion by a grain of sand or similar which caused the pearl to be produced), which cannot escape and irritates the mollusc which retaliates by coating the irritant with layer upon layer of nacreous material. Both marine and freshwater molluscs produce pearls, but the most valuable varieties originate in the pearl oyster of the Persian Gulf. The most highly prized pearls are spherical. When a pearl that has been cut from the shell presents a hemispherical surface, it is sometimes called a bouton pearl. If a solid pearl has an irregular shape, having grown over a rough object, it is known as a baroque pearl. In the jewellerytrade, pearls are commonly known as pear, bell, or drop, according to the shape. Pearl coloration varies widely, the most prized shades being white, black, rose, and cream. River pearls are produced by freshwater mussels in various parts of the world. China is the principal trader in river pearls. Natural, spherical pearls have been cultured successfully since 1920. In this process a mother-of-pearl bead, from three-quarters to nine-tenths of the diameter of the desired product, is introduced into the
pearl oyster. Over a period of years the oyster deposits layers of nacre around the bead. Cultured pearls are not easily distinguished from genuine pearls except by an expert. The technique of producing spherical cultured pearls was developed in Japan, and the culturing of pearls is a major Japanese industry - a small bead of mother of pearl being inserted into the oyster which grows the pearl around the bead. Artificial pearls, in contrast to cultured pearls, are entirely man-made, largely of glass. Research Pearl
Abbreviations
Research Pearl
